Comprehensive structural insights and electrochemical evaluation of rhamnose and salicin for green corrosion protection of carbon steel in acidic medium

This study presents a comprehensive electrochemical and theoretical evaluation of two naturally occurring organic compounds, Rhamnose and Salicin, as green corrosion inhibitors for carbon steel in 1 M HCl. Electrochemical techniques including Potentiodynamic Polarization (PDP), Electrochemical Impedance Spectroscopy (EIS), and Electrochemical Frequency Modulation (EFM) were employed to assess inhibition performance. At a concentration of 1.0 × 10^-3 M, Salicin achieved a maximum inhibition efficiency of 96.10%, while Rhamnose reached 91.91%, as determined by PDP. EIS analysis revealed a significant increase in charge transfer resistance (<i>R</i> _ct) from 19.05 Ω cm² (blank) to 172.27 Ω cm² for Salicin and 121.65 Ω cm² for Rhamnose. The adsorption behavior followed the Langmuir isotherm, with calculated free energies of adsorption of -33.21 kJ mol^-1 for Salicin and -32.59 kJ mol^-1 for Rhamnose, indicating spontaneous mixed-mode adsorption. Density Functional Theory (DFT) calculations revealed that Salicin possesses a lower energy gap (Δ<i>E</i> = 6.321 eV) and higher electron transfer capability (Δ<i>N</i> = 0.943) compared to Rhamnose (Δ<i>E</i> = 8.767 eV, Δ<i>N</i> = 0.783), suggesting superior reactivity and adsorption potential. Adsorption locator simulations confirmed stronger binding of Salicin to Fe(110) surfaces, with an adsorption energy of -230.86 kcal mol^-1 <i>versus</i> -83.58 kcal mol^-1 for Rhamnose. These findings highlight the potential of Salicin as a highly efficient, eco-friendly corrosion inhibitor and demonstrate the value of integrating molecular-level insights into inhibitor design.